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Full text of "Genetic Factors Affecting the Strength of Linkage in Drosophila"

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ZOOLOGY: A. H. STURTEVANT 555 

in two days. This number of eggs therefore has not received the ex- 
posure necessary to produce the change in crossing over. The culmina- 
tion of the two days exposure is to be expected in those eggs so 
situated that 125 to 175 eggs will be laid before them. Such an 
interpretation makes it extremely, likely that the change in the amount 
of crossing over is finally affected in the earliest oocytes, that is, at 
the beginning of the growth period. The above evidence on the time 
of applying the new temperature and the time when the change in 
crossing over occurs, suggests that the crossing over process takes place 
in the stage when the chromosomes of Drosophila are known to be finely 
drawn out threads. 

The decrease in the strength of linkage caused by temperature in no 
way weakens the chromosome interpretation of linkage. It rather adds 
to it considerable support, for it localizes the process of crossing over at 
a period in oogenesis when twisting between homologous threads seems 
possible. The evidence positively establishes the fact that crossing over 
does not take place during the early oogonial divisions, and makes it 
extremely improbable that it occurs at so late a stage in the growth 
period as the thick thread stage favored by Janssens as the chiasmatype. 

bridges, C. B., /. Exp. ZooL, Wistar Inst., Philadelphia, 19, No. 1, July, 1915. 
2 Sturtevant, A. H., these Proceedings, 3, 1917, (555-558). 
5 Cf. Snyder, C. D., Amer.J. Physiol., 22, 1908, (309). 

GENETIC FACTORS AFFECTING THE STRENGTH OF LINKAGE 

IN DROSOPHILA 

By A. H. Sturtevant 

ZOOLOGICAL LABORATORY. COLUMBIA UNIVERSITY 
Communicated by T. H. Morgan, July 5, 1917 

In September, 1913, a wild female Drosophila of a stock from Liver- 
pool, Nova Scotia, was crossed to a male bearing the second chromo- 
some mutant characters vestigial and speck. A single daughter of this 
mating was tested, and gave no crossovers among 99 offspring, though 
vestigial and speck usually show about 37% crossing over. This 
strain has since been bred in very large numbers, and the experiments 
are being continued; but it has seemed advisable to report briefly on some 
of the results obtained. 1 

It has become clear that the original result was due to something in 
the second chromosome derived from the Nova Scotia female. Two 
of her granddaughters and all of her later descendants that were known, 
from linkage, to have received the second chromosome in question gave 



556 ZOOLOGY: A. H. STURTEVANT 

similar results; but those descendants known not to have received it 
gave the 'usual' result. This was true whether the chromosome 
came from the mother (as above) or from the father. And when other 
second chromosome genes were substituted for vestigial and speck, un- 
usual ratios still appeared; but for any combination the result was 
relatively constant, and the combined 'unusual' results give as con- 
sistent a scheme as the 'usual' results. The first column of table 1 
shows the percentage of crossing over in the ordinary females for various 
combinations of second chromosome genes. 2 The second column gives 
similar data for tests of females that carry a Nova Scotia chromosome. 
Figures 1 and 2 are chromosome maps constructed on the basis of these 
two columns. 

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A large number of these experiments have been made with females 
having one Nova Scotia chromosome, and its mate bearing the mutant 
genes black, purple, and curved. Numerous tests have been made in 
order to determine the nature of the crossovers produced. It has been 
found that those crossovers that receive the part of the original Nova 
Scotia chromosome lying to the left of the purple locus still give unusual 
results and transmit the peculiarity to all of their descendants that 
receive this piece. The results produced by such females are not, how- 
ever, the same as those produced when all the Nova Scotia chromosome 



ZOOLOGY: A. H. STURTEVANT 



557 



is present, as is shown in column 3 of table 1 and in figure 3. When 
the other piece of the Nova Scotia chromosome, lying to the right of 
purple (and, in many experiments, that part of it that is also to the 
left of speck) is tested, a still different result is obtained, as shown in 
column 4 and figure 4. 

TABLE 1 



PERCENTAGE OF CROSSING OVER 



Normal 



Cni C r i 



c nl 



'--ill Cn 



Star black 

Star purple 

Star curved .... 

Star speck 

Black purple. . . . 
Black vestigial.. 
Black curved. . . 
Black speck. . . . 
Purple vestigial. 
Purple curved . . 
Purple speck.. . . 
Vestigial speck. . 
Curved speck. . . 
f Total length. . . 



39.3 
40.2 
46.5 
48.3 
5.9 
16.9 
22.7 
49.3 
11.1 
19.0 
46.5 
36.7 
29.8 
94.0 



0.0 
0.0 
X 
0.4 

0.2 
1.2 
1.3 
1.1 
1.2 



1.4 



0.0 

X 
25.9 
49.0 

0.4 

X 
25.6 
48.8 

X 
20.5 
47.4 

X 
35.2 
56.1 



42.4 

45.4 

47.3 

46,0 

5.8 

8.9 

7.5 

9.3 

0.6 

1.7 

2.8 

0.0 

0.2 

50.1 



38.2 
40.6 

X 
49.0 

3.6 

X 

X 
43.5 

X 

X 
42.2 

X 

X 
84.0 



0.3* 

0.3* 

X 
47.4 

0,1 

X 

X 
47.0 

X 

X 
47.4 

X 

X 
47.8 



* These few (6) crossovers are doubtful. None of them were tested; and there is apt to be 
a small percentage of error in classifying Star flies. 

f Calculated as Star black + black purple + purple curved + purple speck in all but the last 
two columns, where purple speck is used. In no case further corrected for double crossing 
over. 



It follows from these results that the original Nova Scotia chromosome 
contained at least two factors causing reduced crossing over, and each 
affecting chiefly the region in which it lies. We may call these two fac- 
tors 'Cni' and 'Cur' (C for crossing over, after Muller, 3 II to dis- 
tinguish them from similar factors affecting other chromosomes, I and 
r for left hand and right hand). 

By appropriate matings it has been possible to obtain females homo- 
zygous for C nr , and a surprising result has been observed: the effect 
produced by C nr when heterozygous disappears when it becomes 
homozygous (see columns 5 and 6, figures 5 and 6). There can be no 
doubt of this result, as it has been obtained repeatedly, and has often 
been checked by tests of the offspring. It has also been paralleled more 
recently by results obtained by Muller 3 (1916) and by me 4 with C ni . 
Similar tests of homozygous C n i have not been possible since no chromo- 



558 ASTRONOMY: F. H. SEARES 

some has yet been obtained that contains both that factor and any 
mutant factor within its "sphere of influence." 

Backcross tests have shown that males of all the above types with 
respect to C n i and C m resemble "normal" males in giving no cross- 
overs at all. 

Included in the above tables are a considerable number of data in- 
volving three or more loci at once; and these agree with the table in show- 
ing clearly that the linear order of the factors established for the usual 
second chromosome is unchanged by the factors under discussion. The 
amount of crossing over is altered, often markedly, and not usually 
proportionately in different regions; but the factors keep their same 
sequence. This result serves to emphasize the importance of consider- 
ing the distances on chromosome maps as only diagrammatic, not as 
representing actually proportionate distances between the genes, al- 
though actual distance is evidently an important factor influencing the 
end result. It does not, I think, in any way weaken the case for the 
chromosome hypothesis, but merely shows, together with the results of 
Bridges 5 (1915) and Plough 6 (1917), that any chromosome map is avail- 
able for purposes of numerical prediction only when the conditions 
under which it was made are duplicated. 

1 Some of the early results were reported at the 1913 meeting of the American Naturalists, 
and brief references have been published by me (1915) and by Muller (1916). 

2 Much of this data has not hitherto been published. It has been collected mainly by 
Dr. C. B. Bridges, to whom I am indebted for permission to use it. 

3 Muller, H. J.;Amer. Nat, Lancaster, Pa., 50, 1916, (193, 284, 350, 421). 

4 Sturtevant, A. H., Zs. Abst. Vererb., 13, 1915, (234). 

e Bridges, C. B., J. Exp. Zool., Wistar Inst. Philadelphia, 19, 1915, (1). 
6 Plough, H. H., these Proceedings, 3, 1917, (553-555). 

FURTHER EVIDENCE ON THE CONCENTRATION OF THE STARS 
TOWARD THE GALAXY 

By Frederick H. Seares 

MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Communicated by G. E. Hale, July 9, 1917 

In a previous communication 1 attention was directed to the striking 
difference in the results for the distribution of the stars with respect to 
the galactic plane found by Kapteyn 2 and by Chapman and Melotte. 3 
An analysis of the counts of stars on photographs of 88 Selected Areas, 
made at Mount Wilson with the 60-inch reflector, gave preliminary 
values for the variation in the totals to magnitude 17.5 which agree well 
with those of Kapteyn.